1. Field of the Invention
The present invention relates to a battery checking apparatus of a camera controlled by a CPU.
2. Description of the Related Art
A conventional camera has a battery check system wherein a voltage detection circuit detects whether or not a power supply voltage is at not less than a predetermined level and supplies a result of the detection to a battery checking input terminal of a CPU, and the CPU controls the camera in accordance with the result supplied thereto. This battery check system is disclosed in, for example, Published Unexamined Japanese Patent Application No. 60-220869. The power supply voltage is usually detecting a closed-circuit voltage when current is caused to flow through a dummy load.
The CPU has a reset terminal for starting a sequence from its initial state when a power supply is turned on. The reset terminal continues to receive a reset signal until the power supply voltage reaches the predetermined value to prevent the CPU from malfunctioning when the power supply is turned on. The reset signal is usually generated from a reset circuit which monitors the power supply voltage.
A camera system using a CPU thus necessitates a voltage detection circuit for checking a battery and a reset circuit for performing a reset operation, and the CPU necessitates at least a terminal for receiving a result of battery check and a reset terminal.
Recently, the batteries of a camera have only to be exchanged with each other every several years. The reset circuit and reset terminal therefore operate only once for several years in common use of the camera.
The number of input/output terminals of the CPU and that of circuits mounted outside the CPU greatly affect simplification and low cost of the camera. It is desirable that an infrequently-used terminal or circuit having a single function, such as the above reset terminal and reset circuit, be reduced in number to the utmost.
If, however, both the voltage detection circuit and reset circuit are connected in common to the reset terminal, the above case where a battery is checked by causing current to flow through a dummy load, has the following drawback.
In power-on reset, when the voltage detection circuit detects that a voltage reaches a predetermined level from 0 level directly after power is turned on, the CPU is started. In battery check, when the voltage detection circuit detects that a power supply voltage is at not more than a predetermined level while current is flowing through the dummy load, it gives an alarm indicating shortage of battery voltage, and stops the current flowing through the dummy load. The voltage then increases and, in this case, the power supply voltage may exceed the detected level. In short, while the CPU is operated when the voltage changes from low to high in the power-on reset, the CPU must be prevented from operating when the voltage changes from low to high in the battery check.